In Preview, it's fairly obvious (since it's in the Tools menu) that you can switch Tools by pressing Command-1 though Command-4. However, there is another less-obvious and non-documented way to switch tools. Just press Option-Right Arrow or Option-Left Arrow.

Hint Options

Click here to return to the 'Another way to switch the active tool in Preview' hint

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Another way to switch the active tool in Preview
Authored by: hibbelig on Feb 06, '06 02:47:59AM

What I miss most is the ability to change between the two marking tools in Preview. While I can use Command-4 to switch to the marking tool, I have yet to find a method to select from the keyboard between the red ovals and the text postits.